Interaction between wave hydrodynamics and flexible vegetation

Abstract

Aquatic vegetation attenuates local currents, dampens wave energy and promotes sedimentation. Its function as shoreline defence has gained strong interest in the recent years, since it may offer sustainable and cost-effective solutions to coastal protection problems. Predictions of wave dissipation by macrophytes require understanding of the hydrodynamics, plant motion and the coupling between the two. The Dynveg model focuses on the interaction between the organisms and the surrounding fluid at the stem-scale. It describes the motion of a blade based on the balance between the parameterized flow-induced loads and the restoring action of the stiffness and the buoyancy of the plant. The present thesis validated its performance for the small deformation regime under oscillatory flows using experimental data. After the validation, the model was used in a one-way coupling by introducing prescribed velocity profiles under varying wave conditions and changing plant properties. The asymmetric plant motion observed in the field by a number of authors was not obtained when depth-uniform oscillatory flows were used as an input. It appeared when linear wave theory was used, before including non-linear effects or the eulerian flow. The results suggest that the driving mechanism of asymmetry in plant posture is the orbital motion, although higher harmonics or the streaming in a canopy may enhance it even further. This may have important implications on the methods use to study plant motions under waves, particularly about the use of U-tubes or depth-averaged models. Neglecting the variations of the velocity along the horizontal coordinate for the tested conditions did not introduce significant errors and in the results, although the differences may be larger for very flexible plants or very energetic waves. Stems with different rigidities were modelled under the same wave condition. Stiffer plants dissipated more energy. Nevertheless, it was found plants with appreciable tip excursions, of about the 20% of its height, could produce the same amount of work than a completely still plant. Longer plants reduced their relative motion with respect to the flow for a longer fraction of the wave period compared to shorter stems, which remained pronated and still over a longer part of the wave cycle and consequently produced more work. The curves of the dimensionless wave-averaged work as a function of the Cauchy number for different plant heights showed a good collapse, which may open the possibility of developing more simplified frameworks to account for wave attenuation in large-scale coastal models.